Steps to reproduce the issue

• Create a new article, contact or newsfeed (maybe others as well) and save once intially
• Check the "Modified Date" and "Modified By"

Expected result

• Either both are not set like in J3 where they are only set after the next save (edit)
• or both are set

Actual result

Only the modified date is set, but the user remains empty until the next save (edit)

Additional comments

I can see where that comes from, since with MySQL strict mode the modified column can't be an empty string. It has to be a valid date or the "nulldate".
However apparently it somehow works for com_fields where creating a field has the same behavior as in J3: The modified date is only populated after the first successive save/edit. I think it's because com_fields, the modified date has filter="unset" in the XML which means it gets nulled during validation.

We should choose a way how we want to handle that date and be consistent across all extensions, either always setting it (but then also the user) or only after an edit (like com_fields).